U.S. Pat. No. 6,908,446 describes an extracorporeal circuit (extracorporeal apparatus) which extracorporeally circulates blood. The extracorporeal circuit includes a venous line and an arterial line which are connected to a patient, a blood reservoir connected on a downstream side of the venous line, an oxygenator connected on an upstream side of the arterial line, and a linkage line which links the blood reservoir and the oxygenator.
When the extracorporeal circuit (hereinafter, simply, circuit) described in the aforementioned patent is used to extracorporeally circulate blood, a priming solution (for example, physiological saline) is first fed into the circuit to prime the circuit. Thereafter, extracorporeal circulation is performed. When such an operation is performed, blood is diluted with the priming solution (hemodilution).
In recent years, a technique of collecting the priming solution as much as possible so as to hinder blood dilution (hemodilution), i.e., an increase in dilution ratio of blood, has been adopted prior to performing extracorporeal circulation. The technique is generally referred to as retrograde autologous priming (RAP). To carry out RAP, a collection bag for collecting the priming solution is connected to the circuit in advance. In this state, blood is drawn from a patient to the circuit in a direction opposite to the direction of the normal flow of blood during extracorporeal circulation. Thus, the priming solution in the circuit is thrust toward the collection bag by the blood. As a result, the priming solution is collected into the collection bag.
As the collection bag, the one formed by fusing the edges of two sheet materials having flexibility is usable. In the collection bag, the respective parts not fused to each other of the two sheet materials (the parts other than the fused part) define the storage space in which a priming solution is stored. Herein, the part of each sheet material defining the storage space is referred to as a “storage space defining part”. The collection bag is configured such that the storage space defining parts are in close contact with each other in such a state that a priming solution has not yet been stored in the storage space, i.e., in an unused state. When the priming solution starts to flow into the storage space from this state, the storage space defining parts are separated away from each other, resulting in an increase in volume of the storage space. Therefore, the pressure in the storage space decreases. At this time, the priming solution is excessively (involuntarily) drawn into (flows into) the storage space from the circuit. Then, the priming solution further flows into the storage space, and the volume of the priming solution in the storage space approaches the maximum volume. The pressure in the storage space increases to such a degree as to thrust the priming solution back to the circuit side, i.e., to hinder the flow of the priming solution into the storage space. At this point, it becomes impossible to collect the priming solution. Thus, the priming solution cannot be collected in the proper quantity when the priming solution is desired to be collected.